Notes from the Field: Port Jackson Sharks and Southern Eagle Rays
Author: Chloe Roberts
Chloe is currently a PhD candidate, Southern Shark Ecology Group at Flinders University
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I’ve been interested in the ocean from a young age, so it’s not surprising that I’ve ended up doing a PhD in marine science. I’m focusing on the diet and movement of sharks and rays around Australia. Lately, my research has taken me to local beaches of Adelaide, South Australia, to catch and sample Port Jackson sharks and southern eagle rays. These are benthic elasmobranchs that feed on small fish and crustaceans. Little is known about their diets and movements in South Australia, so I’ve been catching them, taking some blood and muscle samples, and inserting small acoustic tags to track their movements.
Inserting a small acoustic tag into a ray
Once we’ve caught the shark or ray, we bring them onshore and turn them upside down to induce tonic immobility. This is similar to being anesthetized, so the animal doesn’t respond to pain and allows us to do our sampling. We also pump water through their mouth and out their gills to facilitate their breathing. I insert a small acoustic tag into their peritoneal cavity and then stitch them up with some dissolvable sutures. Muscle is taken using a biopsy probe from their wing area for the ray, and near their second dorsal fin for the shark. Then I take some blood from their caudal vein in the base on their tail.
Taking a blood sample from a Port Jackson shark
Using this blood and muscle, I can determine the diet of the individual using tests called fatty acid analysis and stable isotope analysis. These tests look for specific chemicals in the individual, some of which come from specific dietary items or habitats, so we can determine the diet of the individual based on how much of these chemicals they have accumulated. I’ll be studying how the diet of Port Jackson sharks and southern eagle rays overlap, and how this compares to their movements. I will also perform similar research at sites further away from urbanized, coastal environments to determine how urbanization may be impacting the diet and space use of these species. Coastal cities with large populations come with large storm drains and wastewater treatment plants that discharge into the ocean. Understanding the diet and movement of Port Jackson sharks and southern eagle rays and how these aspects of their ecology vary with the effects of urbanization can help guide management actions for marine pollution.